High-efficiency and pollution-controlling in-situ gasification chemical looping combustion system by using CO2 instead of steam as gasification agent

Zhe Shen ,Zhiyu Huang

1 State Key Laboratory of Oil and Gas Reservoir Geology and Exploration,Southwest Petroleum University,Chengdu 610500,China

2 School of Energy and Architecture,Xi'an Aeronautical University,Xi'an 710077,China

Keywords:CO2 Steam iG-CLC Comparative study

A B S T R A C T Using CO2 as gasifciation agent instead of steam in in-situ coal gasification chemical looping combustion(iG-CLC)power plant can eliminate energy consumption for steam generation,thus obtaining higher system efficiency.In this work,a comparative study of iG-CLC power plant using steam and CO2 as gasification agent is concentrated on.The effects of steam to carbon ratio(S/C)and CO2 to carbon ratio(CO2/C)on the fuel reactor temperature,char conversion,syngas composition and CO2 capture efficiency are separately investigated.An equilibrium carbon conversion of 88.9%is achieved in steam-based case as S/C ratio increases from 0.7 to 1.1,whereas a maximum conversion of 84.2%is obtained in CO2-based case with CO2/C ranging from 0.7 to 1.1.Furthermore the effects of oxygen carrier to fuel ratio(φ)on system performances are investigated.Increasing φ from 1.0 to 1.4 helps to achieve char conversion from 75.9%to 88.9%in steam-based case,by contrast the char conversion can achieve 66.3%–84.2%in CO2-based case within the same φ range.In terms of iG-CLC power plant,recycling partial CO2 to the fuel reactor improves the overall performance.Approximately 3.9%of net power efficiency are increased in CO2-based plant,from steam-based plant.Higher CO2 capture efficiency and lower CO2 emission rate are observed in CO2-gasified iG-CLC power plant,expecting to be 90.63%and 85.18 kg·MW-1·h-1,respectively.

1.Introduction

Because of the clear and incontrovertible evidence related to continuously increasing CO2emission influences planet temperature rise,effective decision-making to design a climate policy is of importance to reduce the irreversible risk of climate change.With regard to fossil fuel-relevant CO2emission as a major contributor,many mitigation options can help mitigate climate change,focusing on innovation and investment in environmental-friendly technologies and utilizing sustainable energy[1].A number of CO2mitigation pathways have been implemented as represented by pre-combustion,oxygen-combustion and post-combustion when fossil fuel is used for power generation[2].However,these methods lead to losses in overall power efficiency owing to the intensive energy penalty[3].Approximately 10%–40%energy penalty will be offered for carbon capture[4].

Notably,coal has been widely used for power generation,accounting for 40%of electricity generated from coal worldwide[5].Nowadays clean coal technologies are mainly addressing on three aspects,i.e.coal combustion process,coal gasification process,and some novel advanced coal conversion technologies[6].In terms of coal combustion process,energy efficiency improvement is considered as the main aim for sustainable development[7].Compared to combustion,coal gasification converts coal into syngas by means of partial oxidation with gasification agents(e.g.steam and oxygen),and the generated syngas can be utilized for power generation or synthesized for chemical products.The main advantage of coal gasification is its ability to achieve lowerenergy-penalty CO2capture because of the increasing CO2concentration in syngas after gasification and water gas shift reaction[8].Furthermore,co-gasification of biomass and coal has been suggested to reduce CO2emissions because of the carbon-neutral biomass[9].Advanced coal gasification processes adopt novel process intensification strategies and conversion process to promote the energy conversion efficiency.Such technologies are within the early demonstration stage,which mainly include high-efficiency membrane based gasification system and chemical looping based approach[5].

Chemical Looping Combustion(CLC)is new and has potential for inherent separation of CO2with zero energy penalty.Basically,CLC reactions occurred in two twin reactors,i.e.a fuel reactor(FR)and an air reactor(AR).One type of chemical intermediates(i.e.oxygen carrier,OC)is used to be circulated between both reactors to alternatively oxidize fuel to CO2and H2O in the FR and to regenerate oxidized OC in the AR[10].At this stage,pure CO2can be easily obtained at the outlet of FR through water condensation[11].Important progresses have been achieved from laboratory testing in TGA and small batch reactor to 1 MWth rig with hard coal by means of in-situ gasification chemical looping combustion(iG-CLC)[12,13].Within this technology,the OCs and solid fuel are physically contacted[14,15].In iG-CLC,the char contained in solid fuel is in-situ gasified by H2O or CO2,subsequently the gasified gases as well as pyrolysis gases directly react with OC to raise a mixture stream consisting of CO2and H2O.However gasification is regarded as the rate-determining step in the iG-CLC[16–19].

A wide range of OCs acceptable for iG-CLC process has been screened previously.A continuous 100 h testing in an interconnected fluidized bed using coal as fuel with intermediate NiO/Al2O3particles witnessed the reactivity deterioration of the NiO oxygen carriers due to the interaction between NiO and support Al2O3to form NiAl2O4[20].Recently a 5 kWth interconnected fluidized bed reactor of CLC driven by coal with hematite as OC was designed and operated.Low gas leakage and no sign of agglomeration were observed and the combustion efficiency and CO2yield reached or exceeded 96.33%and 85%,respectively[21].Notably,the natural ilmenite(FeTiO3)was widely adopted in iG-CLC process owing to its relatively lower price,high mechanical strength and reasonable reactivity in reducing atmosphere.With ilmenite as OC,both pre-oxidation and activation processes were necessary to improve the reactivity of ilmenite[22,23].Cuadrat et al.investigated the ability of ilmenite as OC to burn coal in a 500 Wth iG-CLC unit.Higher operating temperature was beneficial to char conversion from 15%to 82%as well as combustion efficiency from 70%to 95%with the temperature variation in the range of 870–950°C[24].In parallel,a 100 kWth CLC plant exploring the performance of ilmenite was successfully run in CHALMERS[25].Recently the world's largest ilmenite-related CLC plant of 1 MWth was tested in Darmstadt[12].

In iG-CLC system,both CO2and steam have been suggested as gasification agents for converting char into syngas(mixture of CO and H2).For saving energy,utilization of CO2as gasification agent can eliminate the external energy requirement for steam generation.As suggested by Brown et al.,CO2instead of steam can be used as gasification agent for gasifying highly reactive solid fuels,such as low-rank coals or biomass[26].Mendiara et al.recommended that replacing steam by CO2as gasification agent in low-rank based iG-CLC system could in part save huge amount of energy associated with steam generation[27].

Despite all the comments mentioned above attempting to explain the benefits of utilizing CO2as gasification agent in iG-CLC process,there are very few work available in the literature to systematically analyze the strengths and weaknesses with replacing steam by CO2in the iG-CLC plant.Two major concerns should be focused:(1)if the thermodynamic performances(e.g.carbon conversion rate)maintain optimistic with the shifting the gasification agent from steam to CO2;(2)Though utilization sequence-ready CO2(from iG-CLC process)as gasification agent eliminates the energy penalty associated with raising steam,it still doubts whether the energy penalty related to CO2recompression is reasonable to render CO2-gasified iG-CLC plant attractive.In this study,we therefore conduct a comparative analysis between CO2-gasified and steam-gasified iG-CLC process to demonstrate the ability of the former option from thermodynamic assessment.In this comparative analysis we focus on the carbon conversion rate,the ability to reach for auto-thermal operation,CO2capture efficiency as well as syngas compositions for the parametric analyses.Furthermore this investigation is fascinated into iG-CLC process coupled power plant to comparatively analyze the steam and CO2-gasified iG-CLC power plant.With these efforts,we aim to identify thermodynamically potential trends of using CO2as gasification agent in the iG-CLC plant as well as pointing out the main barriers and shortcomings and provide recommendations to the community.

2.Process Description

Fig.1 presents a simplified diagram of iG-CLC power system using two candidate gasification agents,i.e.steam and CO2.A complete iGCLC power system is mainly composed of two islands,namely a reaction island and a power island.The former one is mainly composed by an air reactor and a fuel reactor,and the latter one consists of a combined cycle and a heat recovery steam generation unit[28].

Coal mixed with gasification agent,i.e.steam originated from Power Island[shown in Fig.1(a)]or CO2derived from sequestration-ready CO2[shown in Fig.1(b)]is fed to the fuel reactor where coal undergoes three-step reactions.

Initially,the coal is thermally devolatilized with a rapid pyrolysis.This step is demonstrated to be fast enough,therefore coal pyrolysis is considered to be happened immediately.

Subsequently,the remaining char is transformed into syngas incorporation with either steam or carbon dioxide.The gasification step is a rate-determining step for the whole iG-CLC process[29].

With carbon dioxide the gasification occurs[30]:

And with steam as[30]:

The water-gas shift(WGS)is also capable of affecting the composition obtained in the fuel reactor,and shown as[30]:

Eqs.(3)and(4)can be integrated into a single expression as[31]:

Finally,the products from coal gasification as well as volatiles matters react with the oxygen carrier in the fuel reactor,fully oxidized to CO2and H2O.

The reactions shown in Eqs.(1)–(3)are highly endothermic,and the heat required to maintain autothermal operation is supplied by both gas combustion reactions and the sensible heat of the heated OCs from the AR.

Clearly in Fig.1,due to the indirect contact between coal and air,a FR-outlet material stream,mainly consisted of CO2and H2O,is finally obtained.And vapor condensation benefits separation of CO2without extra energy consuming.On the other side,for the purpose of regeneration of reduced OCs,oxidation of reduced ones with air is also necessary in the AR.

Typically,the regeneration reaction is significantly exothermic and the heat released during this process is used for heating both oxidized OCs and reduced air.

Finally,the excessive thermal energy contained in both flue gases are collected by means of combined cycle for power generation.For the steam-gasified iG-CLC power plant,the necessary amount of steam for gasification are extracted from power island,by contrast the required CO2as gasification agent to convert char is originated from sequestration-ready CO2stream.

3.Simulation Description

In this section,detailed schematics of two above-mentioned cases are focused on.Simulations of both cases are conducted by application of Aspen Plus(v 7.2)already-existing formulations and embedded modules.The PR–BM(Peng–Robinson with Boston–Mathias)method is chosen as the proper method owing to its suitability for gas processing application[32].

In this work,the coal mass flow rate is fixed to be 10 kg·s-1,which is to match a medium-size 160 MWth CLC power plant,since a conceptual design of a 100 MWth iG-CLC plant has been previously conducted[33].During simulations,heat and pressure losses in each subunits are neglected because of this work belongs to an initial attempt to verify the possibilities of shifting gasification agent from steam to CO2.Reactions involved in the simulations are subjected to be equilibrium except for the coal pyrolysis process,meaning the reaction residence time is supposed to be long enough to obtain both chemical and phase equilibrium.Coal pyrolysis is assumed to be happened instantly as soon as coal is fed into the FR,which is a generally simulation-adopted method for dealing with solid fuel pyrolysis[34,35].Though positive or negative interactions between ash and OCs have been previously reported[36],this influence strongly depends on the ash content,the nature of ash,the experimental conditions and the diversities of oxygen carriers[36–38],and this influence is generally margin.Therefore,the ash is assumed to be inert and no interaction between ash compounds and OCs is expected from this work.Furthermore because the main composition in char is experimentally examined to be carbon,therefore residual char after coal pyrolysis is assumed to be composed by 100%carbon.

Fig.2.Simulation diagram of iG-CLC power plant using steam as gasification agent.

3.1.Simulation of iG-CLC power plant using steam as gasification agent

Fig.2 presents the simulation diagram of an iG-CLC power plant using steam as gasification agent.Beginning from coal preparation,the prepared coal with suitable size for gasification will undergo three steps as explained previously.Herein,a low-rank coal,lignite,is selected as raw material for investigation because it has been previouslydemonstrated that active coal(low-rank)is benefited to CO2-char gasification.The proximate and ultimate analysis of lignite is shown in Table 1.

Table 1 Proximate and ultimate analysis(wet basis)and lower heating value of lignite[17]

In the simulations,three reactors in series are considered to simulate the reactions between coal and ilmenite,as shown in Fig.3.In the first step,coal is subjected to thermal pyrolysis to produce volatile matter and residual char.Coal pyrolysis happened at a relatively lower temperature and it will take few minutes to finish.In the Aspen Plus simulator,a Ryield model,based on specifying reaction yields of each component,is utilized to decompose coal into its substantial element compounds accounting on the ultimate analysis of coal material.The composition of each products is calculated through an embedded FORTRAN subroutine.

In the second step,the residual char and unburnt char from a carbon stripper in conjunction with volatiles are fed into char gasification step,where char gasification with gasification agent(i.e.CO2and steam)happened to generate syngas.An in-built RGibbs model in Aspen Plus is considered to calculate chemical and phase equilibrium based on minimizing Gibbs free energy of all components expected to obtain equilibrium[39].The un-gasified char and ash are separated from gaseous compounds and sent into a carbon stripper for promoting overall carbon conversion.

Finally,the gases,mainly composed by volatiles from coal pyrolysis and syngas generated from char gasification,are subjected to combustion process with oxidized oxygen carrier.In this work,activated ilmenite is considered as one of suitable OCs with a composition of 22.0%Fe2O3,38.5%Fe2TiO5,34.0%TiO2and 5.5%inerts(mass basis).The oxygen transportation capacity is 3.3%[40].The activated ilmenite is capable of fully oxidizing hydrocarbons into CO2and H2O.After that,the reduced ilmenite is similarly sent into a carbon stripper and simultaneously a mixture stream,which is mainly constituted with CO2and H2O,is generated.Worth noting is there does exist some impurities in FR-Flue gas,such as fuel-N compounds,fuel-S compounds,few unconverted CO and H2,which will affect the quality of the captured CO2stream.And the influence of impurities in captured CO2stream on CO2compression and storage is not carried out in this study,and more information can be found elsewhere[27,41–43].

Fig.3.Detailed simulation of FR reactor.

The reduced ilmenite,unburnt char and inert ash are together introduced into a carbon stripper which is aimed at separating unconverted char from OCs and sending it back to the FR to improve the overall carbon conversion efficiency,with simultaneously avoiding carbon emission in the downstream AR[44].The carbon stripper is typically referring to a bubbling fluidized bed-type reactor with steam as fluidizing gas[11].And the separation is based on density difference of the particles.A carbon stripper is modelled by a Sep model in Aspen Plus with an assumption of 95%carbon separation efficiency.

The separated ilmenite OCs together with small fraction of unseparated char(5%of carbon)are introduced into the AR.In such a nature,it offers a possibility to re-oxidize reduced ilmenite into its activated state by the compressed air(1.5 MPa).It should be noted that both the FR and AR are operated at 1.5 MPa and the operating temperature of AR is consistent with 1050°C[45].However the FR is operated at adiabatic condition(at approximately 900°C)in order to examine the possibility of autothermal operation.

Both the FR-Flue gas(mainly constituted by CO2and H2O)and the AR-Flue gas(mainly constituted by N2)contain high thermal energy,which can be further collected through a first gas turbine and then a heat recovery steam recovery(HRSG)unit.The gas turbine is based on a Compr model that existed in Aspen Plus with restricted isentropic efficiency of 88%and mechanical efficiency of 98%[46].A HRSG unit is utilized to recover excessive heat from exhaust gas released from the gas turbines,simultaneously generating three-level reheated steam.The three-level steam pressure adopted in this work accounts for 8 MPa/1.5 MPa/0.3 MPa and the steam temperature is heated to 450°C.The pressurized middle-temperature steam is used for power generation according to three-level steam turbines.In the last step,the exhaust steam(at pressure of 5000 Pa)is cooled down in a condenser.

3.2.Simulation of iG-CLC power plant using CO2 as gasification agent

Fig.4 presents a simulation diagram of iG-CLC power plant using CO2as gasification agent which offers a possibility to reduce energy consumption for external steam generation.To carry out a comparative analysis,the fuel feeding conditions remain unchanged.

The difference between two cases is mainly attributed to distinct gasification agent.In this diagram,the cooled CO2stream(after water condensation)is partially transported back to the FR.The separated CO2stream is compressed to 1.5 MPa,meeting FR operating pressure.Subsequently the compressed CO2stream is preheated to 450°C,keeping consistent with the feeding temperature of steam gasification agent,by means of heat exchanger.

4.Results and Discussion

Fig.4.Simulation diagram of iG-CLC power plant using CO2 as gasification agent.

In this section,a comparative study between CO2-gasified and steam-gasified iG-CLC process is first presented by examining the potentials of using CO2gasification agent.Notably,pure CO2(not recycling CO2from captured CO2stream,which always contains some impurities)at temperature of 450°C,is considered to assess this possibility.Subsequently a comparison between steam-gasified and CO2-gasified iG-CLC power plant is focused on to evaluate the potential benefits of increasing power efficiency by using CO2gasification agent.

4.1.Parametric analysis for iG-CLC process

4.1.1.Effect of steam to carbon ratio(S/C)and CO2 to carbon ratio(CO2/C)

Steam to carbon ratio(S/C,defined by the molar ratio between steam flow entered into gasifier and carbon content in coal)is an essential parameter to determine the iG-CLC performances by using steam as gasification agent.Insufficient S/C results in reduction of char conversion,further lowering CO2capture efficiency.On the contrary,higher S/C requires external energy for rising steam.

CO2to carbon ratio(CO2/C,defined by molar ratio between recycled CO2flow and carbon content in coal)is another main parameter to determine the performances of CO2-gasified iG-CLC process.

Fig.5.Effect of steam to carbon ratio(S/C)and CO2 to carbon ratio(CO2/C)on FR temperature and carbon conversion.

The amount of steam and CO2added into the FR highly affects the char conversion and operating temperature of FR,as shown in Fig.5.An equilibrium carbon conversion of 88.9%is achieved as the S/C ratio increases from 0.7 to 1.1,however FR temperature is observed to be decreased with S/C from approximately 920°C to 900°C.Both linear relationships associated with S/C are shown in this figure.Increasing S/C dramatically promotes the char gasification reaction with steam[Eq.(3)],however due to its endothermic,temperature dropping is expected.Notably char gasification with steam is highly endothermic.Though middle-temperature steam from the HRSG(450°C)provides large amount of heat required for char gasification,it cannot offset the energy consumption for gasification,therefore slight decrease in FR temperature is observed.

By contrast,when CO2is used as gasification agent,fast temperature dropping is anticipated within the same range(i.e.0.7–1.1)as decreasing from 915°C to 896°C.This phenomenon is caused by the higher endothermic char gasification with CO2[Eq.(2)]in comparison with steam-char gasification[Eq.(3)].Interestingly,when CO2/C changes from 0.7–0.85,the char conversion is superior to that in case of steam-gasified iG-CLC process within the same range(S/C=0.7–0.85).The reason for this phenomenon might be explained as follows:lower CO2/C ratio acquires higher carbon conversion due to the insignificant inhibition influence from water-gas shift reaction,and consumption of CO benefits for Boudouard reaction[Eq.(2)].However higher CO2/C ratio(＞0.85)shows an unavoidable inhibition influence on water-gas shift reaction,thus promoting reversed water-gas shift,which is favorable for CO production.The increasing CO forbids the conversion of char with CO2(inhibition of Boudouard reaction).As a consequence,in the case of CO2-gasified iG-CLC process,lower char conversion is observed at higher CO2/C ratio.

On the other hand,syngas composition from char gasification step is studied owing to different syngas compositions essentially determine the reactivity of ilmenite OCs.As reported by Muhammad et al.[47],higher reactivity of H2with activated ilmenite OCs was disclosed in comparison with CO.The effect of S/C and CO2/C on syngas composition is shown in Fig.6.As the S/C increases from 0.7 to 1.1,the H2concentration in syngas remains almost unchanged,and slight decrease of CO concentration is observed,whereas CO2concentration increases from 4.4%to 5.8%.Increasing S/C accelerates char gasification reactions with steam[Eq.(3)],benefited to H2and CO generation.However the reversed water–gas shift reaction(endothermic)is enhanced at high temperature range,and hydrogen as reactant will be consumed.Consequently,hydrogen composition is almost remained unchanged.The Boudouard reaction[Eq.(2)]is reduced with temperature dropping(increasing S/C results in temperature drop,as shown in Fig.5),as a result,increasing CO2concentration,corresponding with dropping with CO concentration,is probably resulted.

Fig.6.Effect of steam to carbon ratio(S/C)(a)and CO2 to carbon ratio(CO2/C)(b)on syngas composition(dry basis).

Different from usage of steam as gasification agent,CO instead of H2is the main component in syngas owing to different gasification strategies.According to Fig.6(b),by increasing CO2/C,the CO concentration exhibits an optimistic trend towards CO2/C,as increasing from 62.6%to 63.8%due to the enhancing char gasification reaction with CO2.Unfortunately H2concentration is decreasing from 20.5%to 14.7%which is caused by the inhibition influence on water-gas shift reaction as explained above.The CO2composition is generally gained due to the continuous feeding of CO2.

The final CO2emissions from the AR are ascribed by the unburnt char coming from a carbon stripper slips into the AR.Fig.7 presents the effect of S/C and CO2/C on CO2emission and CO2capture efficiency.

Clearly,strongly dependent relationships between S/C and CO2emission as well as CO2capture efficiency are observed.Increasing S/C benefits carbon conversion(as shown in Fig.5),therefore CO2emission reduction from 21.7 t·h-1to 6.7 t·h-1is expected.Simultaneously,a maximum CO2capture efficiency of 88.9%is obtained.

At CO2/C ＜0.85,CO2emission from the AR in case of CO2-gasified iGCLC process is lower than that in steam-gasified process.The more carbon converts in the FR,the less CO2emissions happen in the AR.Following the changing trend of char conversion with CO2/C(as shown in Fig.5),it has been observed CO2emission reduction as well as higher CO2capture efficiency.After this value,no significant benefit is observed when CO2is employed as gasification agent.

Fig.7.Effect of steam to carbon ratio(S/C)and CO2 to carbon ratio(CO2/C)on CO2 emission and CO2 capture efficiency.

Obviously,both higher S/C and CO2/C benefit carbon conversion and overall iG-CLC performances.A higher S/C requires a considerable amount of heat to produce steam,greatly decreasing in the overall energy efficiency.Meanwhile a higher CO2/C represents an enhancing compression work related to recycling CO2stream from transportready CO2stream to the FR.In this work,in terms of both S/C and CO2/C,a moderate value of 1.1 is recommended in this work[48].

4.1.2.Effect of oxygen to fuel ratio

The circulating rate of ilmenite OCs can significantly affect the heat balance of the overall iG-CLC process.Herein,the effect of oxygen to fuel ratio(φ)is defined as the availability of oxygen in the flow of oxygen carrier divided by the oxygen required to fully convert the fuel to CO2and H2O[49].It can be inferred that φ=1 corresponds to the stoichiometric conversion of coal into CO2and H2O.

The effect of φ in the range of 1.0–1.4 on FR temperature and carbon conversion in both cases is shown in Fig.8.The heated ilmenite OCs(at 1050°C)provide sensible heat to heat the FR,therefore this influence is mainly attributed to the temperature-dependence relationship as the φ studied here is in the range of 1.0–1.4,which is above stoichiometric value of 1.In another words,at this stage,increasing φ is immune to gasification reactions due to a fully oxidation of reducing gases which has been maintained.

Fig.8.Effect of oxygen carrier to fuel ratio(φ)on FR temperature and carbon conversion using steam and CO2 as gasification agent.

In terms of steam-gasified iG-CLC process,increasing φ from 1.0 to above-stoichiometric 1.4 favorably benefits FR temperature rising(easy to obtain desired operating temperature),increasing from approximately 850°C to desired 900°C.With the increasing operating temperature,better char conversion is expected due to its endothermic,as increasing from 75.9%to 88.9%in the temperature interval of 50°C.

The effect of φ on FR temperature is almost independent with gasification agents.Replacing steam into CO2does not make a big difference towards FR temperature.However char conversion is elatively sensible to φ(in the range of 1.0–1.4),as it changes from 66.3%to 84.2%in CO2-gasified iG-CLC process.

The effect of φ on syngas composition using steam and CO2as gasification agent is shown in Fig.9.Enhancement of φ mainly replies on improving operating temperature.In the case of steam-gasified iGCLC process,an increase in φ benefits endothermic char gasification[Eqs.(2)and(3)],as a consequence,an increase in CO concentration associated with a decrease in CO2concentration is expected.Due to the important role of reserved water–gas shift reaction(endothermic),continuous consumption of H2is unavoided.

Fig.9.Effect of oxygen carrier to fuel ratio(φ)on syngas composition(dry basis)using steam(a)and CO2(b)as gasification agent.

By contrast,with increasing φ from 1.0 to 1.4,CO concentration rapidly increases from 55.3% to 63.8%, resulting from higher temperature favors char gasification with CO2[Eq.(2)],with decreasing CO2concentration from 28.1%to 19.4%.However H2composition shows trace increase with φ owing to the contribution of insignificant water–gas shift reaction.

The effect of φ on CO2emission and capture efficiency in both steamgasified and CO2-gasified iG-CLC system is presented in Fig.10.The results indicate that increasing the amounts of ilmenite OCs into the FR causes a considerable CO2capture efficiency increment from 75.9%to 88.9%in steam based case and from 66.2%to 84.2%in CO2based case.

Obviously higher oxygen carrier to fuel ratio(φ)improves the overall iG-CLC process performances.However the iG-CLC process is not economic interestingly to be operated at high φ ratio owing to the additional cost on transportation OCs as well as solid separation.In this work,φ is recommended to be 1.4 in both process.

4.2.Comparative study of both cases

Table 2 presents the calculated results concerning the comparative performances of iG-CLC power system between using steam and CO2as gasification agent.It is worthy to note that pure CO2stream(without any impurities)is utilized in previous parametric analysis to investigate the possibility of using CO2as gasification agent.However in this section,CO2used for gasification comes from recycling CO2stream,which contains certain fraction of impurities,i.e.fuel-S(SO2,H2S),fuel-N(mainly N2)and incomplete combustion gases(CO and H2).The CO2/C ratio is still considered as 1.1,which is defined by the ratio of molar flow of CO2in recycling gas to molar flow of carbon in coal.

Obviously,the compressing work in both cases makes big difference due to the required work for recycling CO2stream in CO2-gasified iGCLC power system,with approximately 3.71 MW increment from steam-based case.As a consequence,total energy input is distinct in two cases,with 235.18 MW in steam-based case as against 238.89 MW in CO2-based case.Remarkably,in case of CO2-based iG-CLC power plant,due to the unnecessariness of extracting steam for gasification,about 6.32 MW ST output electricity is gained in comparison to iG-CLC power plant using steam as gasification agent.The net electricity output from CO2-based case is expected to be 65.87 MW,accounting for 3.8%higher than that in the steam-based case with a lower net electricity output of 63.38 MW.

It is not surprise to obtain a higher system efficiency in CO2-gasified iG-CLC power plant,i.e.40.54%of net power efficiency,in contrast with steam-gasified iG-CLC power plant.Approximately 3.9%of net power efficiency is increased in CO2-based plant,from steam-based plant.

Fig.10.Effect of oxygen carrier to fuel ratio(φ)on CO2 emission and CO2 capture efficiency using steam and CO2 as gasification agent.

Table 2 Performance of both steam-gasified and CO2-gasified iG-CLC power plant

Higher CO2capture efficiency is observed in CO2-gasified iG-CLC power plant,expecting to be 90.63%.However this result is different from previous parametric analysis(Section 4.1),where CO2capture efficiency using CO2as gasification agent(at CO2/C=1.1)is lower than that using steam as gasification agent(at S/C=1.1).This phenomena is caused by the impurities in CO2stream(e.g.sulfur compounds)is capable of bringing sensible heat to the FR,resulting in higher operating temperature of the FR and correspondingly higher carbon conversion.

From the perspective of environmental aspect,CO2-based iG-CLC power plant has a reduced CO2emission of 85.18 kg·MW-1·h-1due to its higher capture efficiency.By comparison,the net power output related to captured CO2accounts for 1.22 MW·h·t-1in CO2-based iG-CLC power plant,whereas about 0.02 MW·h·t-1decrement is observed in steam-based iG-CLC power plant.

4.3.Further research needs

IG-CLC process poses as one of the most promising technology for CO2capture to mitigate greenhouse gas effect.From the obtained thermodynamic analyses,possibility of replacing steam into CO2as gasification agent offers higher power efficiency.Future research needs are still necessary to address this possibility.

(1)We herein put forward a thermodynamic model to comparatively assess steam and CO2-gasified iG-CLC plant.However it should be noted in practice the char gasification step(especially CO2as gasification agent)is favored to be controlled by kinetics,which is mainly restricted by the residence time and reaction rate in the FR.Future performances of real iG-CLC plant are possible to be close to thermodynamic equilibrium conditions with promotion of carbon stripper efficiency,increasing the residence time of char in the FR(e.g.prolonging the reactor length),and probably using the catalysts to accelerate reaction rate[13].

(2)Generally,adaption of higher CO2/C ratio in CO2-gasified iG-CLC plant exhibits reduced char conversion and CO2capture efficiency in comparison with using steam as gasification agent.One possible solution is employment of low-rank coal or biomass(lower carbon content)as raw material to narrow down these reductions.

(3)As explained previously,lower CO2/C shows a higher char conversion and CO2capture efficiency.Therefore possibility of multistage gasification,i.e.first employment of certain amount of CO2to pre-gasify char and following employment of relatively large amount of steam to gasify the residual char,or adapting mixture gasification agent(CO2and steam)can save amount of energy required for steam generation.

5.Conclusions

This work carried out a thermodynamically comparative comparison between steam and CO2-gasified in-situ coal gasification chemical looping combustion(iG-CLC)power generation plant.In light of the obtained results,both higher steam to carbon ratio(S/C)and higher CO2to carbon ratio(CO2/C)contribute to char conversion.When S/C and CO2/C change in the same range of 0.7–1.1,the char conversion obtains a maximum value of 88.9%in steam-gasified iG-CLC process as against a maximum value of 84.2%in CO2-gasified iG-CLC process.When the oxygen carrier to fuel ratio(φ)changes in the range of 1.0–1.4,an increment of char conversion from 75.9%to 88.9%is observed in steam-gasified iG-CLC unit,as against that from 66.3%to 84.2%in CO2-gasified unit.In terms of iG-CLC power plant,approximately 3.9%of net power efficiency is increased from CO2-based plant,compared to steam-based plant.Higher CO2capture efficiency is observed in CO2-gasified iG-CLC power plant,with 90.63%capture efficiency as against of 88.90%capture efficiency in steam-based iG-CLC power plant.